In the production of contact lenses the lenses are typically automatically inspected for defects in the optical zone as well as in the peripheral portion of the lens, since the peripheral portion and in particular the lens edge greatly affects the wearing comfort that is experienced by the user. Contact lenses which are defective either in the optical zone or in the peripheral portion are rejected, at least if predetermined threshold values for identifying such defects are exceeded.
In a known mass production process of contact lenses a viscous starting material is dispensed into a female mold half, the mold is then closed with the aid of the male mold half, and the starting material in the mold is then exposed to UV-light, so that the starting material between the shaping surfaces of the male and female mold halves is crosslinked and/or polymerized to form the contact lens. Subsequently, the mold is opened and the lens so formed is loosened from the mold half to which it sticks by the application of water. Also, the water serves to wash away any excess starting material which has not been crosslinked and/or polymerized. The wet contact lens is then gripped and later in the process the peripheral portion of the lens, in particular the edge of the lens, is automatically inspected. Typically this is done with the aid of image analysis, which itself makes use of a high contrast image (dark background, bright representation of the lens edge).
However, due to the fact that the contact lens the edge of which is to be inspected is still wet (e.g. water droplets adhere to the lens), the automatic inspection system may detect a lens as being defective although it is not. The water droplets cause the lens edge to appear darker in a portion of the lens edge where a small water droplet adheres than in a portion where no such water droplet is present. Since dark portions at the lens edge are interpreted as representing a defect of the lens edge, the lens may be rejected although the lens edge indeed is not defective, thus unnecessarily increasing the reject rate. Previous attempts have been made to improve this misinterpretation by drying the contact lens by means of application of pressurized air with the aid of nozzles having discrete openings, however, this has still resulted in portions at the lens edge which are represented darker so that lenses have been rejected as being defective although they are not.
Accordingly, it is an object to overcome these drawbacks and to provide suitable measures to decrease the number of contact lenses which are rejected as being defective at their lens edge although indeed they are not.